The boo lexer turns the text script into a set of tokens, and the boo parser converts the tokens into an abstract syntax tree (AST). I'll show examples of this later on. Each step after the parser is basically just working on this AST and transforming it. At the end, the AST is used to produce raw .NET assembly code (IL). The steps that happen after the parser step:

Here is a script that spits out the AST after each step in the compile process, in xml or boo format. It doesn't show everything going on, because only the visible changes to the AST structure itself are shown. See the source code for each step and for the boo compiler for more details.